1. Field of the Invention
The present invention relates to a method for dynamic generation of synthetic images with automatic detail level, as well as to a device for implementing this method.
2. Discussion of the Background
The real-time image synthesis machines used in flight simulators can generate almost fully realistic images, in particular by large-scale use of photographic textures. However, the maximum number of polygons which can be displayed at each image cycle is still their main limitation.
Unfortunately, this number is found to be very poorly used in some situations. This is because the same attention is paid to the distant facets as to the close facets, irrespective of the observer's visibility distance. However, during calculation, the pertinence in the image of the distant facets is very small compared to those found close to the observer. Some attempts have been made in the past to simplify the long-distance landscape. However, it has been found that they are not very effective and are too constringent in terms of the database generation.
It is possible to visualize real terrains in synthetic images by virtue of the existence of altimetric maps resulting from radar or satellite observations. These altimetric data are generally in the form of a two-dimensional grid giving the altitude at each point.
The terrain model (algorithms and data structures) which manages these altimetric data should take into consideration the following three requirements which are essential, in particular for aircraft pilot simulators:
Faithfulness
The characteristic aspects of the relief (peaks, valleys) are very important visual references for pilots and influence the quality of their training and their decisions during a mission. Faithfully representing the ridge lines is therefore an essential criterion for any cartographic model.
Economy of Information
For equal precision, the number of polygons representing a given terrain directly influences the response times of a real-time simulator (rendition, collision, roll, intervisibility, etc.). Since the roughness of a terrain is not regular, the mesh generation should adapt to the relief, and be coarse in zones with constant slope and fine in undulating parts.
Speed of Generation
Since the simulation databases may cover thousands of km.sup.2, their generation cost is directly linked with the use of high-performance algorithms which make it possible to integrate the various data sources (planimetry, altimetry, photometry) in a minimal amount of time.
Since the databases of aircraft simulators are in general very extensive, the number of facets representing the terrain is considerable. However, the visual system can display in real-time only a few thousands of facets. In order very rapidly to eliminate those which are not in the field of view, region pre-truncation is carried out. During the modelling of the database, the terrain is partitioned into rectangular zones referred to as regions. Simple pyramid-box intersection calculations make it possible to select the visible regions and thus eliminate a very large number of facets.
This division is also very useful if the database cannot be loaded into memory in one block. It is then sufficient to load only the regions lying in a sphere which is centred on the observer and has a radius equal to the visibility distance. This local database is updated as the observer moves: the regions leaving the sphere are dumped and replaced by those entering it. It is thus only memory space which limits the size of the databases.
For smaller visibility distances (&lt;10 km) and a moderately undulating terrain, region pre-truncation is found to be sufficient in order to guarantee the image calculation frequency. Beyond this, the workload management of the visual system remains problematic. Overloading the visual system with polygons results in image jumps following cycle overflows. This situation is scarcely acceptable for a real-time flight simulator. The only remedy currently available is to simplify the zones of the database where the display "jams". This solution is expensive and not very practical. Furthermore, for large visibility distances, the relief is simplified far too much.
Only sophisticated detail level algorithms can greatly reduce the number of facets displayed without degrading the quality of the image. These algorithms have arisen from the following observation: the pertinence of a polygon of the terrain, that is to say the number of pixels which it occupies on the screen, decreases as the polygon becomes further away. If nothing is done, some of the graphics power of the machine is wasted on clipping, projecting, texturing, etc. polygons which, in the end, only occupy one or two pixels on the screen. The entire difficulty therefore consists in simplifying a terrain seen by a mobile observer without compromising the pertinence of the image.
Faced with the mathematical and algorithm complexity, no simulator manufacturer has integrated a device of this type truly effectively.
A first approach consists in precalculating various detail levels for each region and in switching from one level to another in real-time. Unfortunately, this method has many drawbacks:
Dependence on the Regions
Since the regions here fulfill the role of boundaries between detail levels, the quality of the switching depends on their sizes.
Memory Requirement
The memory requirement limits the number of detail levels per region and penalizes the dynamic loading of the database. Furthermore, this number varies as a function of the relief of each region.
Excessively Abrupt Switching
As the number of detail levels of a region decreases, the abruptness of the switching increases. These image artefacts are a great problem for the pilot.
In order to remedy these problems, hierarchical terrain models have been envisaged. A hierarchy of detail levels within the same data structure (quad tree, Delaunay pyramid) only stores the changes for moving from one detail level to another, which gives a significant memory saving. The tree structure is followed in order to select the triangles to be displayed as a function of the required precision.
Three major drawbacks remain
Overall Detail Level
The construction of the successive detail levels is based solely on a precision criterion, independently of an observer's position. The order in which the points appear is fixed, whereas the pertinence of the points does indeed vary as a function of the observer's position.
Modifications to the Terrain are Impossible
These hierarchical data structures are rigid and do not allow any modification to the terrain in real-time. It would be necessary to reconstruct the entire tree structure, which is too expensive.
Additional cost of generating the database.
The generation of the database remains something which is very expensive. The constraints linked with real-time complicate the modelling of the scene.